In the past electromagnetically actuated (EM) relays and switches have been employed for use in higher power rated circuits having power ratings of from a few volts to 5 KV or more and with corresponding current ratings of from 50 amperes to several hundred amperes or greater. These EM relays and switches while satisfactory in many respects are bulky, heavy, slow responding and tend to develop excessive arcing and sparking across the contacts during operation while opening and closing due to their operation in an ambient air atmosphere.
For a number of practical reasons, due to their bulk, weight and out gassing properties, known EM relays and switches can only be operated in air and cannot be enclosed within a protective gastight enclosure that is evacuated.Iadd.. .Iaddend.Operation in air enables prolonged arcing which is induced during opening and closing of the contacts of such EM relays and switches. This is due to ionization of the air gaseous medium in the space between the contacts as they open or close so that the operating life of such EM devices in service is severely reduced .[.and.]. .Iadd.which .Iaddend.adds greatly to maintenance problems and expense. Further, EM devices dissipate considerable heat and cannot be upgraded in performance since they are not voltage (capacitor) operated. Lastly, operation of EM device contacts in air induces oxidation of the contact surfaces .[.and.]. .Iadd.which .Iaddend.can greatly increase contact resistance.
Relays and switches which use piezoelectric drive elements have a number of advantages over their electromagnetic (EM) driven counterparts. For example, a piezoelectric driven relay or switch requires substantially lower current and dissipates very little power during operation to open or close a set of load current carrying contacts in comparison to an electromagnetic driven device of the same power rating. Additionally, piezoelectric driven switching devices have very low mass, require less space and introduce less weight into circuit systems with which they are used. Lastly, piezoelectric driven switching devices may have very short actuation times and thus respond much faster than do their EM counterparts. Thus, fast acting switching is possible with smaller and lower weight devices which dissipate less power and generate less heat than does an EM relay or switch of the same power rating.
A number of different piezoelectric ceramic switching devices have been offered for sale in the past having a variety of different configurations. One of the more popular and prevailing structural approaches in these known devices.[.,.]. is referred to as a bimorph bender-type piezoelectric ceramic switch device which employs two adjacent piezoelectric plate elements mounted side by side and having conductive electrodes coating their outer surfaces and sharing a common conductive inner surface to form a bimorph bender member. A known commercially available bimorph bender-type piezoceramic switch is described in an application note copy-righted in 1978 .Iadd.and .Iaddend.published by the Piezo Products Division of Gulton Industries.[.,.]. Inc. located in Metuchen, N.J. and Fullerton, Calif. Another such prior art piezoceramic switching device is described in U.S. Pat. No. 2,166,763 issued July 18, 1939 for a "Piezoelectric Apparatus and Circuits". In the intervening years since 1939, piezoceramic bender-type switching devices have been the subject of widely-spread efforts to improve their characteristics. This is evidenced by a relatively large number of patents which have issued in the intervening years such as U.S. Pat. No. 2,714,642 issued Aug. 2, 1955 for a "High Speed Relay of Electromechanical Transducer Material"; U.S. Pat. No. 4,093,883 issued June 6, 1978 for "Piezoelectric Multimorph Switches".Iadd.; .Iaddend.and U.S. Pat. No. 4,403,166 - issued Sept. 6, 1983 for "Piezoelectric Relay with Oppositely Bending Bimorph". Such piezoceramic bender-type switching devices also have been described in a textbook entitled "Manual of Electromechanical Devices" by .[.Douglass.]. Douglas C. Greenwood, editor, published by McGraw-Hill Book Company and copyrighted in 1965.
Heretofore, piezoelectric ceramic bender-type relays have been described as being employed in a variety of circuits which involve switching of low power rated electrical circuits (i.e., signal level circuits with voltages less than 20 volts and corresponding milliamp range currents). Virtually no commercially available relays have been sold. Also, to date no serious effort has been made to increase the power rating of piezoceramic bender-type relays. A key requirement for a bender actuated relay is the ability of the short gap that forms between the bender-actuated switch contacts as they open (or close) to withstand voltages impressed upon it by the external circuit to which the device is connected. To increase the voltage withstandability of this gap between the contacts after extinction of current flow, it is advantageous to choose an ambient atmosphere such as a vacuum or an inert gas or high dielectric strength atmosphere such as nitrogen and argon or sulfur hexafluoride (SF.sub.6), and the like. In such protective vacuum or inert gaseous atmospheres, the gap space between the contacts can attain as high a dielectric as is possible. This is an important consideration regardless of whether the circuit to be switched operates .Iadd.at .Iaddend.a few volts or 5000 volts since the ability of the contact gap-space to withstand whatever voltage is required after current extinction while the gap spacing is short, translates into a shorter time needed to achieve that gap and consequent higher operating speeds and capability of higher voltage operation.
Relays (which were not piezoelectric in nature) have been operated in a vacuum according to a report in a prior publication entitled "High Voltage Switching with Vacuum Relays" by Ronald V. Tetz and Robert W. Hansen in a paper presented in 1965 at a relay conference conducted by the Institute of Electrical and Electronics Engineers (IEEE). In this publication there is no clear disclosure of the mechanical details of construction of the switch or how it was arranged so that contacts .[.were.]. .Iadd.could be .Iaddend.operated in a vacuum. Further, as of the present date so commercially practical high power vacuum relays have appeared on the market. In addition, at a conference held in 1978 by the IEEE and identified as the Holm Conference, a paper was presented entitled "Electret Driven Electrical Relays" by D. Perino, G. Dreyfus and J. Lewiner-pages 441-446 wherein an electret, not piezoelectric, type relay device .Iadd.was .Iaddend.operated in a vacuum enclosure .[.and suitable for use.]. at signal levels (less than 20 volts) .[.is.]. .Iadd.as .Iaddend.disclosed on page 445. However, electrets due to their nature are hard to bake out during evacuation and further do not hold their charge well so that prolonged usage would not be possible. To the knowledge of the present inventors there has been no previous publication or use of piezoelectric ceramic switching devices mounted and operated within a protective gastight enclosure either in a vacuum or in a protective inert gaseous atmosphere .[.and suitable.]. for operation at higher power levels.